How does castling work in Chess960?

The king and rook always end on the same squares as in standard chess. Castling short puts the king on g1 and the rook on f1; castling long puts the king on c1 and the rook on d1. Only the squares they start from change. All squares between the start and end positions must be empty, the king may not pass through check, and neither piece may have moved.

Why is the king always between the rooks in Chess960?

Because castling would otherwise be undefined. Requiring the king to start between the two rooks guarantees there is always one rook on each side to castle with.

How many Chess960 positions are there?

Exactly 960. The bishops must sit on opposite colours, the king must stand between the rooks, and Black mirrors White. Those constraints leave 960 legal arrangements, numbered 0 to 959. Position 518 is standard chess.

Is Chess960 harder than normal chess?

It is harder to prepare for and easier to start. There is no opening theory to memorise, so beginners are not punished for not knowing lines, but strong players lose the preparation advantage and must calculate from move one.

Chess960 rules, and the castling rule everyone gets stuck on

Chess960, also called Fischer Random, is ordinary chess with the back rank shuffled. Same pieces, same moves, same goal. Two things change: where the pieces start, and how castling is described.

What actually changes

Before the game, the eight back-rank pieces are shuffled into one of 960 legal arrangements. Pawns are untouched. Black's back rank mirrors White's exactly, so neither side gets a structural advantage. Every other rule of chess is unchanged: pieces move the same way, en passant works the same way, promotion works the same way, and checkmate is still checkmate.

The three constraints that make it 960

Bishops on opposite colours. One light-square bishop, one dark-square bishop, exactly as in standard chess.
The king between the rooks. There is always one rook to the king's left and one to its right.
Black mirrors White.

SP 12 · QBNNRKBR

Position 12. The bishops sit on b1 and g1, one on a light square and one on a dark one, and the king stands between the rooks on e1 and h1. Every legal arrangement satisfies both conditions, which is what leaves exactly 960.

Those constraints leave exactly 960 arrangements, numbered 0 to 959. Position 518 is ordinary chess. You can step through all of them with the position generator.

Castling in Chess960

This is the part that trips people up, and the rule is simpler than it looks:

The king and rook always finish on the same squares as in standard chess.

Castling short (kingside): king ends on g1, rook ends on f1.
Castling long (queenside): king ends on c1, rook ends on d1.

Only the squares they start from are different. Everything else follows from that: all squares the king and rook travel through must be empty (apart from those two pieces themselves), the king may not start in check, pass through check, or end in check, and neither the king nor that rook may have moved earlier.

The cases that feel wrong but are legal

Because the starting squares vary, castling can look strange:

The king may not move at all. If the king already stands on g1 and the rook on h1, castling short simply moves the rook to f1.
The rook may not move at all. If the rook is already on f1, only the king travels.
The king may move rightwards to castle long. With the king on b1 it travels to c1, while the a1 rook jumps over it to d1.

None of these are exceptions. They all fall out of the one rule: finish on g1/f1 or c1/d1.

How castling is written

Notation is still O-O and O-O-O. In computer formats you will meet X-FEN or Shredder-FEN, where castling rights name the rook's file (for example HAha) instead of the usual KQkq, because with shuffled pieces "kingside" is ambiguous until you know where the rooks are.

Is Chess960 harder?

It is harder to prepare for and easier to start. There is no theory to memorise, so a beginner is not punished for not knowing lines. A strong player loses their preparation edge and has to calculate from move one. That is why it is such a good training tool: it removes memory from the equation and leaves only what you can actually see.

Train on it

Recognising a fork on b1 instead of f7 is a different skill from knowing the pattern. The free Chess960 puzzle trainer drills exactly that, with engine-verified positions from real games, and the books hold a thousand at a time.